You just put the DLLs in the same directory as your executable and create a qt.conf with the content:

[Paths]
Prefix=.\
Plugins=.\

This way you ensure that the Qt system will pick up the plugins you might use from the correct directory (for example sqldrivers directory for the SQL plugins). Of course, you can customize these paths according to your needs, this is the simplest solution...

Make sure you pick the correct DLL from the Qt directory to which your application links to.